Hearing aid devices developed for patients with hearing impairment use a gain controller to amplify the sound collected by a microphone, and output a loud sound from a speaker, which greatly improves the patient's sound recognition.
However, in processing that merely amplifies the sound collected by a microphone with a gain controller and outputs a loud sound from a speaker, the patient's hearing may still not be sufficiently improved, particularly when it comes to understanding conversation.
One reason for this is that speech is made up of vowels (low tones) and consonants (high tones). Specifically, most patients find it particularly hard to hear sounds in the high tone frequency band, that is, consonants, and this inability to hear consonants properly is believed to hinder a proper understanding of conversation.
One possible way to deal with this problem is to raise the amplification of the gain controller. However, when the amplification is raised, the sound pressure (the volume, or sound level) of vowels also increases, resulting in a state in which consonants end up being buried in these vowels (a masking state). As a result, there is the risk that the patient's hearing will not be sufficiently improved in terms of understanding conversation, as mentioned above.
In view of this, the following Non-Patent Literature 1 proposes that a first hearing aid worn on one ear function for low tone use, and that a second hearing aid worn on the other ear function for high tone use. That is, vowels in conversation are low tones and are therefore picked up by the first hearing aid, while consonants in conversation are high tones and are therefore picked up by the second hearing aid, and processing by the brain forms these into a single sound. Consequently, the user can hear conversation more easily.